Note: Descriptions are shown in the official language in which they were submitted.
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REFRACTORY HEAT-INSULATlNG SLABS
This invention relates to re-Fractory,
heat-insulating slabs for use in lining tundishzs,
methods of making the slabs and tundishes lined
with ths slabs.
In the continuous casting of metals,
e.g. steel, mol-ten metal is poursd from a ladle
into a continuous casting mould via an intermediate
vessel which acts as a constant head reservoir
and is called a tundish. The tundish has a metal
floor and sidewalls and one or more outlet nozzles
set in the floor or a sidewall. To protect the
metal floor and walls of the tundish from the effects
of molten metal it is usual to line the interior
of the tundish with a relatively permanent lining,
often made of bricks. The tundish may additionally
be provided with an inner, expendable lining of
refractory, heat-insulating slabs. This is
described in Sritish p`atent specification 1364665
and is highly advantageous.
Acccrding to the present invention a
refractory, heat-insulating slab for use in the
innerJ expendable lining of a tundish comprises
a slurry-formed facing (to face molten metal
in thE tundish), which comprises magnesium oxide,
inorganic binder but substantially no organic
matter and has a combined water content not
exceeding 2% by weight, and a different backing,
which comprises refractory filler, and binder
and has a permeability of at least 10 AFS ~American
- 30 Foundryman's ~ociety) units, the facing and backing
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having been joined -together during formation o-f
the facing or backing.
The magnesium oxide in the -Facing o-F a
slab of the invention i9 highly refractory, e.g.
high temperature calcined Magnesite such as
dead burnt magnesite.
Slabs of the invention have a very
important advantage in that there is little or
no tendency for a significant amount of hydrogen
to be picked up by molten steel from slabs of the
inveation present in an inner, expendable lining
of a tundish through which the molten steel passes.
It is well known to treat molten steel to reduce
its hydrogen content to an acceptable level before
supplying the steel to a tundish. However, in
accordance with the present invention it has been
app.eciated that steel may pick up a significant
amount of hydrogen from an inner, expendarle lining
of a tundish; by the ~se of slabs of the invention,
this tendency can be reduced.
By having substantially no organic Matter
- in the facing the risk of steel picking up
hydrogen -From such matter is minimised. In this
context the term 'organic matter' is used to
signify hydrogen-containing organic matter and the
presence in the facing of a proportion of carbon
of organic origin, e.g. coke~ is not excluded.
The minimisation of organic matter in the
facing virtually eliminates one source of hydrogen
that might be picked up by steel. However, the
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facing is slurry-formed and, as the slurry-
forming method involves de-watering an aqueous
slurry of ingredients including a binder ln a
permeable mould and then heating the damp article
obtained to dry it and -to set the binder, this
could lead to the presence in the facing of some
water, a source of hydrogen that might be picked
up by steel. The slurry-forming method has, how-
ever, many advan-tages and the heating, which
preferably involves use of a temperature of about
180C, usually serves in practice to reduce the
free water content of the article to a very low
level. The facing of a slab of the invention
preferably contains substantially no free water.
In the facing of a slab of the invention
the combined water content does not exceed 2%
by weight, preferably it is not greater than 1%
and more preferably it is no greater than 0.50%.
It has been appreciated in accordance with the
invention that, depending on the ingredients,
slurry-formed slabs may contain a significant
proportion of combined water because of the
formation of hydrated matter during the processing
of the ingredients. It has also been appreciated
that the hydrated matter may be stable at
temperatureswell in excess of those normally used
e.g. 180C for drying slurry-formed slabs and
setting the binder and thus the hydrated matter is
liable to lead to hydrogen being picked up by
molten steel in contact with the slab.
It is well known that magnesium oxide
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becomes hydrated on contact with water to form
magnesium hydroxlde and the latter is not
dehydrated by heating until 2 temperature as
high as about 415C is reached. However, it has
been found in accordance with the invention
that by careful choice of the magnesium oxide
a high proportion of magnesium oxide can ~e
includsd in the slurry-formed facing and the
combined water content of the facing still kept
1û low without any need to use high temperatures
to decompose hydrated matter and drive off the
water. The magnesium oxide used in making the
facing preferably has a hydration value of 1.7 or
less, more preferably 1.0 or less~ most preferably
0.2 or less.
'Hydration value' as referred to in this
specification is determined by allowing a sample
of the material to be tested to stand in cold
water for 24 hours, drying the residue at a
temperature of 180C for four hours, weighing the
dried residue and heating the dried residue at
1000C to constant weight. The hydration value
is the weight loss caused by the heating at 1000C
expressed as a percentage o-F the weight after
the drying at 180C.
There is a wide variety of standard
methods for determining the hydration value of
magnesium oxide. As an example, the British
Standard method of 8S 1902, Part 1B, involves
contacting a sample with steam at 100C -For 5
hours. The method in question in the present
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-- 5 - FS 1225A
specification was used in view o-F its relation to
the conditions used in maklng and drying slurry-
formeo articles and the fact that the facing
is subject to high temperatures in use.
The facing o-F a slab of the invention
preferably comprises 75 to 95% by weight of
refractory -Filler and the magnesium oxide
preferably accounts for all or most of this.
Any refractory filler in addition to the magnesium
1û oxide is preferably one or more of chromite,
alumina, zirconium silicate, olivine, silica,
zirconia and high alumina aluminosilicates.
Part of the filler e.g. 10% by weight may be
carbonaceous matter such as crushed graphite
electrode scrap, thereby enhancing erosion resist-
ance but slightly increasing thermal conductivity.
The facing preferably comprises refractory
fibre, preferably in an amount of 1 to 10% by
weight. The refractory -Fibre preferably comprises
aluminosilicate fibre or calcium silicate fibre
e.g. slagwool. Inclusion of 1% by weight or more
of refractory fibre helps to impart advantageous
strength and thermal insulating properties to the
facing and avoidance of more than 10% by weight
of refractory fibre helps achievement of good
erosion resistance.
The amount o-F inorganic binder in the facing
is pre-Ferably 2 -to 10iO-by weight. If the amount
of inorganic binder exceeds 10% the -Facing may be
unduly brittle. rhe inorganic binder preferably
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comprises an alkali metal silicate, e.g. sodium
silicate and in this case the SiO2:Na20 weight
ratio is preferably in the range of 2.5-3.7:1
and typically has a weight ratio of 3.35:1, such
material being readily available commsrcially.
Although preferrsd in the present invention, a
disadvantage of an alkali metal silicate binder
in a single layer tundish lining slab is its
tendency to migrate to the surface during drying.
If the amount of alkali metal silicate binder
exceeds 10% by weight there is a risk that the
concentration of binder, due to migration, at
the permanent refractory interface, will be
su-fficiently high to promote alkali attack of the
permanent refractory with corresponding premature
failure thereof.
However, the present invention avoids
this problern by smploying comparatively low
levels of alkali metal silicate binder in the
facing and providing a di-fferent backing which
effectively prevents alkali attack of the
permanent refractory located behind the different
backing layer of slabs of the present invention.
Accordingly, any binder migration of the facing
~5 will only reach the front face of the backing
but will not reach the permanent refractory.
The facing preferably comprises 3 to 7%
by weight o-f an alkali metal silica-te as described
above. I-f the amount is less than 3% the strength
of the facing may be less than is desirable whilst
if the amoun-t is greater than 7% the re-fractoriness
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of the facing may be unduly reduced and there may
be a tendency to pick=up significant amounts of
atmospheric moisture.
Howsver, the backing layer of the present
- 5 inventlon may be formulated in order to provide
adequate supporting means for the facing layer
when the binder present in the facing is less
than 3%. In this way no incidence of brittleness
nor reduced refractoriness will occur. Furthermore,
the backing layer so formulated provides the
facing layer with a measure of protection from
mechanical shock during installation and transit.
In addition to the alkali metal silicate
the binder in the facing may contain a bonding
clay7 prsferably in an amount not exceeding 5%
by weight. The bonding clay is of value for
maintaining the strength of the facing when the
facing is at a high temperature, especially
where the facing is exposed above the level of
molten metal and slag in the tundish.
The facing is substantially free from
- hydrogen-containing organic matter and preferably
no such matter is present. However, up to a
total of 0.25% by weight of such matter e.g. in
the form of organic binder and/or fibre may be
tolerated, depending on the permeability o-F the
facing. The risk of hydrogen pick-up may occur
if the facing is insufficiently porous to permit
the escape via the bacl<ing o-r any hydrogen gases
formedr
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The density of ths facing is preferably
from 1.4 to 2.0 g/cm . A-t lower densities the
erosion resistance may be less than is desired
and thus a thicker facing may be needed. At higher
densities, the facing may have an undesirably high
initial chilling effect on rnolten metal.
The fact that a slab of the invention
has a backing, different from the facing, in
addition to the facing enables particularly
advantageous combinations of properties to be
achieved; e.g. the backing is more permeable than
the facing which facilitates the escape of
hydrogen bearing gases; the backing may be of
lower density than the facing which contributes
to a more lightweight slab which is easier to
handle during manufacture, transportation,
installation etc.; the backing may be hydrophobic
in nature thereby reducing the tendency for the
slab to absorb moisture from the atmosphere;
the backing may be rendered more resilient than
the facing thus offering improved impact resist-
ance in use as a tundish lining; the backing may
be stronger than the facing and thus acts as a
supporting means for an inherently weaker facing.
The backing comprises refractory filler
and a binder and may consist of these e.g. 90%
by weight filler and 5% by weight binder and 5%
by weight organic and/or inorganic fibre. The
filler may be any of those mentioned for the facing
but it is usually not preferred to include
carbonaceous filler in the backing as this
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- 9 - FS 1225A
increases the thermal conductivity. Other
fillers that may be used include silica, e.g.
silica flour, and refractory silicates including
aluminosilicates. The refractory silicates may
bs simple silicates, e.g. olivine, or complex
silicates such as aluminosilicates and these
may he minerals or reclaimed materials e.g.
fire clay grog. Lightweight refractory fillers
e.g. expanded perlite and calcined rice husks
may be used in the backing.
The binder in the backing is pre-Ferably
organic. Examples of suitable organic binders
are starch and resins e.g. phenol-formaldehyde
and urea-formaldehyde resins. It has been found
that organic binders provide the backing with
more resilience than otherwise would be the case.
This is particularly true in the case of phenol-
formaldehyde resin binders.
The backing may include fibre and this
may be refractory and/or organic. Suitable
refractory fibres are exemplified by alumino-
silicate fibres and calcium silicate fibres,
e.g. slagwool. Paper is suitable as an organic
fibre.
The backing preferably comprises a layer
comprising 60 to 95% by weight of refractory
filler, O to 20% by weight of refrac-tory fibre,
O to 10% by weight of organic fibre and 2 -to 15%
by weight of binder.
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The psrmeability of the backing layer
may be about 10 AFS and preferably is greater
than 20 AFS, the permeability of the facing layer
is preferably less than 5 AFS.
The density of the backing is pret`erably
in the rangD of 0.65 to 1.4 g/cm . At lower
densities the backing may be substantially
compressed during use and initially good heat-
insulation properties consequently reduced to an
unsatisfactory level. At high densities the
heat-insulating properties o-f the backing may be
inadequate unless the backing is unduly thick.
The magnesium oxide in the facing is
highly refractory but tends to be associated with
relatively high densities and only moderate heat-
insulation properties. However, by use of a more
heat-insulating backing, good heat-insulation can
be provided by the slab as a whole. The nature
of the backing can readily be so chosen as to
provide good heat-insulation, e.g. by making the
backing of low density, because the backing does
not need to have the erosion resistance of the
facing.
If it is more important that the entire
slab should have good erosion resistance than that
the slab should provide particularly good heat-
insulation~ then the backing is preferably of
high density and preferably includes a refractory
filler, especially magnesium oxide, that promotes
erosion resistance. In this way, even if, a~ter
a tirne, the facing is entirely eroded away, good
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~ FS 1225A
erosion resistance can be provided by the backing.
In use, the backing of a slab of the invention
rapidly becomes sufficiently hot for the hydrogen
o-F any hydrogen-containing mat-ter to be driven
off as hydrogen-bearing gases, and these can
escape preferentially through the backing and
into the atmosphere rather than through the facing
and into the molten metal. Accordingly, even
if the facing is liable to be entirely eroded
away, the backing does not need to be of low
hydrogen content.
If the function of the facing is chiefly
to provide a low hydrogen content layer to face
the molten metal and the backing is of substantial
erosion resistance, the facing may be relatively
thin e.g. 5 mm and the backing relatively thick
e.g. 25 mm. If, however~ the facing is to provide
all the erosion resistance desired for the slab
and the backing is to provide good heat-insulation
but is of relatively low erosion resistance, the
facing and backing may be of generally similar
thickness e.g. 15 mm each.
- As already stated, the facing is slurry-
fnrmed and this can be achieved by de-watering an
aqueous slurry of the ingredients in a suitably
shaped permeable mould and subsequently heating
the product to dry it and to render the binder
effective. The backing can then be formed on top
of the facing by a slurry process or by rnethods
known ~or forming shapes of foundry sand e.g. core
shooting. Preferably, however, the backing is
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formed first and an aqueous slurry of the facing
ingredients then injected above or beneath it
and de-watered and the slab then heated to dry it
and to render effective the binder in the -Facing.
The binder used in the backing may be one that
can be hardened at ordinary ambient temperatures
e.g. it may be a resin that is hardened by use
of a catalyst.
Whilst the slabs of the invention are
adapted to cause little or no pick-up of hydrogen
by steel passing through the tundish, it should
be appreciated that some pick-up of hydrogen by
the steel may result from other causes e.g. water
present in any refractory cement exposed to the
steel in the tundish. Accordingly, the use of
refractory cements should be avoided or
minimised or it should be ensured that any such
product is well dried before the tundish is used.
The invention includes not only the
slabs themselves but also methods of making them
as described above and tundishes having an inner,
expendable lining of the slabs.
The invention is illustrated by the
following Examples.
EXAMPLE 1
The following ingredients in the
percentages (by weight) specified were formed
into a first aqueous slurry:
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Ingredient %
.
silica flour 11
silica sand 80
calcium silicate -fibre 2.6
paper fibre 2
phenol formaldshyde resin 3.1
urea-formaldehyde resin 1.3
The slurry was de-watered in a permeable
mculd shaped to form a slab.
The following ingredients in the
percentages ~by weight) specified were formed into
a second aqueous slurry:
Ingredient
magnesium oxide 88
(hydration value 0.12)
aluminosilicate Fibre 3
sodium silicate 6
powder ~ Si2: Na20 weight
ratio 3.35:1)
ball clay 3
The second slurry was introduced into the
mould over the layer obtained by de-watering the
first slurry and was de-watered through that layer.
The matter in the mould was then removed as a
damp two-layer slab and was heated at 180C to dry
it and to harden the binder in each of the layers.
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The two layers of the slab obtained as
described above adhered toge-ther well and the
first layer deposited i.e. the backing had a
thickness of 16 mm and a density of 1 g/cm and
the second layer deposited i.e. -the facing had
a thickness of 14 mm and a density o-f 1.6 g/cm .
The first layer had a permeability of 35 AFS and
the second layer had a permeability of 5 AFS.
EXAMPLE 2
Example 1 above was repeated with the
exception that the second aqueous slurry was formed
as follows:-
Ingredient
,
magnesium oxide 91.9
(hydration value 1.0)
aluminosilicate fibre 3.0
sodium silicate powder 5.0
(SiO2:Na20 weight ratio
3.2:1~
polyester fibre 0.1
The first layer deposited i.8. the backing
had a thickness of 15 mm and a density of 1 g/cm
and the second layer deposited i.e. the facing
had a thickness of 15 mm and a density of 1.7 g/cm .
1he first layer (the backing) had a permeability of
35 AFS and the second layer (the facing) had a
perrneability o-f 3 AFS.
